Pity the gardener whose once lush basil plant suddenly isn't looking so tasty. Usually easy to grow, basil's many varieties ensure it has culinary versatility. The freshly picked herb is frequently added to salads, paired with melted mozzarella and used as a key ingredient in tomato-based dishes like soups, sauces and pastas. But problems, particularly diseases and pests, can plague basil.
Fusarium wilt is a soil-borne fungus that infects the roots and then moves up the stem, preventing water from moving through the plant, according to the Ohio State University Extension. It causes leaves located lowest on the plant to yellow, curl and wilt, and asymmetric growth. Foliage eventually begins to die, killing seedlings and stunting older plant growth, according to the University of California Agriculture and Natural Resources. Seeds taken from infected plants may also be infected with the disease, making management necessary at greenhouses.
Like fusarium wilt, infected soil causes root rot. The fungus, which proliferates best in cool, moist soil, attacks the roots and can spread within seven to 10 days, making them mushy and unable to absorb water and nutrients, the University of Minnesota Extension Service says. Stem rot may also develop once roots are attacked. Overwatering, poor drainage, inadequate sunlight and plant crowding can cause it.
Downy mildew is a newer disease for basil that was first observed in the U.S. in 2007, Cornell University's "Vegetable MD Online" explains. The most noticeable symptom is plant yellowing, which can be misdiagnosed as a soil nutrition issue. It eventually eats away at leaves, causing dark spots similar to sunscald. The light-colored, downy mildew only appears on the undersides of leaves, making it harder to detect. It can spread by seed, infected leaves and wind-carried spores.
Basil is very susceptible to gray mold, which often attaches to plants after stems have been cut for harvesting on rainy days, the Clemson University Cooperative Extension says. Infection must occur very soon thereafter, because stems heal after 48 hours. It attacks by moving down the plant stem and into leaves and secondary buds. Leaves drop off and the plant dies if the infection enters the main stem.
Basil "has a reputation as an insect repellent," according to the Herb Society of America, but can still be attacked by insects like Japanese beetles, grasshoppers, slugs, aphids and spider mites. Various methods can be used according to the type of bug affecting the plant. Whiteflies can be deterred with a blast of water, while slugs can be kept at bay with 5-inch copper fencing that reacts with the animals' body chemistry by giving it an electric charge. Fabric covers during the day can also prevent infestations.